Recalibrating apparatus and method

ABSTRACT

A method and apparatus for recalibrating analytical instruments such as atomic absorption and inductively coupled plasma spectrophotometers comprises the use of coupled piston pumps and valves in a fluid conduit system to obtain a sample value (h) during one half of a pump cycle and standard gradients (S1-S6) during the other half cycle. The piston pumps are actuated by cams of selected profiles to obtain a predetermined flow rate from each pump, the flow from each pump being selectively combined to construct standard gradients from two or more standard solutions and to effect autoranging of the standards in response to a previously determined sample value.

BACKGROUND OF THE INVENTION

THIS INVENTION relates to the recalibration of sensors and instrumentsused for chemical analysis.

Flow based methods of analysis require periodic recalibration withstandards. Generally, several external standards (typically between twoto five depending on the characteristics of the sensor or instrumente.g. linear or non-linear) are measured at intervals between a group ofsamples, but in all periodic recalibrations, a compromise exists. Veryfrequent recalibration improves reliability, but at the expense ofsample throughput. Infrequent recalibration increases the risk of an outof tolerance condition between recalibrations, requiring the samplemeasurements to be reworked.

U.S. Pat. No. 5,080,866 describes a cyclic flow based analyticaltechnique (Discontinuous Flow Analysis) where the ratio of a reagent tosample is varied during the measurement half of the cycle, and oneexample of the technique shows how an internal recalibration can beperformed with a single standard and stepped flow profile on everysample. This overcomes the problem of the frequency of recalibrationinherent in the use of external standards. However, the concentration ofthe internal standard relative to the sample has to be within certainlimits so that the accuracy of the recalibration is not impaired. If thesample and standard are close in concentration, the step changes aresmall, consequently the resolution may be insufficient for an accuraterecalibration. If the sample and standard are considerably different inconcentration, plateau responses for each step may not be achieved dueto the detector response time, or diffusion effects.

It should be noted that in all cyclic analytical techniques which usepiston pumps, the cycle for each pump comprises two complementaryhalf-cycles: a forward stroke whereby fluid is expelled from the pump,and a return stroke whereby fluid is drawn into the pump. Themeasurement of the sample takes place during one of the half-cycles.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a rapid, effectiveinternal recalibration for fluid delivery systems, in particular, thoseemploying a piston pump fluid delivery system in which recalibrationrequires a minimal number of standards, and which provides arecalibration between successive samples.

A further object of the present invention is to provide a recalibrationsystem which can auto-range the recalibration to suit the precedingsample.

The present invention entails the realization that substantial benefitsin line with these objectives can be achieved by utilizing a novelrecalibration apparatus in which a reproducible recalibration profilebetween two standards is formed in the half cycle complementary to thesample measurement half cycle, and the measurement of the sample in thefirst half of the cycle is compared to the recalibration profile of thetwo standards in the second half of the cycle. Additional standards maybe employed so that the recalibration can be auto-ranged to suit thepreceding sample.

According to one aspect of the invention there is provided arecalibrating apparatus comprising:

a pump means for delivery of a fluid sample, reagent or sample/reagentmixture to a fluid junction;

first and second piston pump fluid delivery systems for delivery atfirst and second flow rates respectively of first and second fluidstandards to the fluid junction;

conduit means providing fluid flow communication between the fluidjunction and a sensor means;

mixing means to mix the first and second fluid standards upstream of thesensor means relative to a fluid flow direction;

sensor means responsive to a fluid condition of the fluid sample,reagent or sample/reagent mixture and responsive to a fluid condition inthe first standard, said second standard or mixtures thereof;

flow rate control means operatively coupled to at least one of the firstor second piston pump fluid delivery systems for controllably varying aflow rate thereof to produce a plurality of ratios of the first andsecond flow rates in accordance with a sequence of functional steps,each of which steps is defined by at least one of a distinct flow rateratio, a series of distinct flow rate ratios, a distinct gradient offlow rate ratios, or a series of distinct gradients of flow rate ratios;

wherein the pump means and the first and second piston pump fluiddelivery systems operate in reciprocal modes of a pumping cycle so thatin one half cycle the pump means delivers the fluid sample, reagent orsample/reagent mixture to the sensor means while the first and secondpiston pump systems refill with first and second standards respectively,and in the other half cycle the first and second piston pump systemsdeliver the first and second fluid standard and mixtures thereof to thesensor means while the pump means reloads with sample or sample/reagentmixture;

and wherein the condition of the sample or sample/reagent mixturedetected by the sensor is compared to the condition of the first andsecond fluid standards and/or mixtures thereof detected by the sensor toeffect calibration of the sample.

The pump means for delivery of a fluid sample sample/reagent mixture maycomprise a positive displacement pump.

Alternatively the pump means for delivery of a fluid sample orsample/reagent mixture may comprise a differential pumping means adaptedto aspirate a sample and/or a sample/reagent mixture.

Suitably the mixing means is located in the conduit means between thefluid junction and the sensor means.

The recalibrating apparatus may comprise valves controlling respectiveports associated with the first and second piston pumps and means forsynchronizing the operation of the valves.

Preferably, the flow rate control means is adapted to producesubstantially pulse free operation of the first and second piston pumps.

Suitably, the flow rate control means comprises cam means operativelyassociated with the first and second piston pumps.

Alternatively the flow rate control means may comprise an electrical orelectro-mechanical control means such as a linear actuation device orstepping motor operatively associated with the first and second pistonpumps.

If required, the recalibrating apparatus may include one or moreadditional piston pump means for delivery of a further fluid standard tothe fluid junction.

The recalibrating apparatus may include electronic control means toselectively actuate valves associated with the first, second oradditional piston pump means, the electronic control means beingoperatively coupled to the sensor means to effect, in use, autorangingof fluid standards associated with the first, second and additionalpiston pump means.

If required the sensor means may vaporise the sample or sample/reagentmixture.

Suitably the sensor means comprises an atomic absorptionspectrophotometer or an inductively coupled plasma spectrophotometer.

According to a further aspect of the invention there is provided amethod of recalibrating a chemical analysis apparatus, the methodcomprising the steps of:

delivering a fluid sample or sample/reagent mixture via a fluid junctionto a sensor means responsive to a fluid condition of the sample orsample/reagent during a first half of an operational cycle;

delivering first and second fluid standards and mixtures thereof via thefluid junction to the sensor means during a second half of anoperational cycle;

controllably varying at least one of a flow rate of the first and secondfluid standards to produce a plurality of ratios of the flow rates ofthe first and second fluid standards in accordance with a sequence offunctional steps each of which is defined by at least one of a distinctflow rate ratios, a distinct gradient of flow rate ratios, or a seriesof distinct gradients of flow rate ratios;

mixing the first and second fluid standards upstream of the sensor meansrelative to a direction of fluid flow through the sensor means;

sensing a fluid condition of the sample or sample/reagent mixturearriving at said sensor means during the first half cycle andsubsequently sensing a fluid condition of the first and second fluidstandards and mixtures thereof to effect analysis of the sample orsample/reagent mixture;

wherein the analysis is determined by the relationship between thesensed condition of the fluid sample or sample/reagent mixture and thesensed condition of the first and second fluid standards and mixturesthereof.

If required delivery of the fluid sample or sample/reagent mixture maybe effected by positively pumping the fluid sample or sample/reagent tothe sensor means.

Alternatively delivery of the fluid sample or sample/reagent mixture tothe sensor means may be effected by aspiration of the fluid sample orsample/reagent mixture with differential pumping means.

The the sequence of functional steps may include sequentially a distinctflow rate of the first fluid standard, a distinct gradient of flow rateratios between the first and second fluid standards, and a distinct flowrate of the second standard.

Alternatively, the sequence of functional steps may includesequentially, a distinct flow rate of the first fluid standard, a seriesof distinct flow rate ratios between the first and second fluidstandards, and a distinct flow rate of the second fluid standard.

Suitably, the fluid condition of the sample or sample/reagent mixture inthe first half cycle is compared to a recalibration profile constructedfrom sensed fluid conditions of the first and second fluid standards andmixtures thereof, the recalibration profile being constructed during thesecond half cycle.

If required, one or more additional fluid standards may be selectivelyintroduced into the sensor means during the second half cycle to effectautoranging of the fluid standards.

According to yet another aspect of the invention there is provided afluid flow chemical analysis apparatus comprising a recalibrationapparatus according to a first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood and readilycarried into effect, preferred embodiments and exemplary methods ofoperation, will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a combination block diagram of a configuration ofrecalibrating apparatus for carrying out the method of the inventionwhere differential pumping is used to aspirate the sample in the samplemeasurement half-cycle.

FIG. 2 is a combination block diagram of a configuration ofrecalibrating apparatus for carrying out the method of the inventionwhere positive pumping only to the sample and/or sample reagent mixturesis used in the sample measurement half-cycle.

FIG. 3 shows schematically a sensor response to a sample and threestandard calibration curves.

FIG. 4 shows schematically a sensor response to a sample and a steppedcalibration curve.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, during the sample measurement half-cycle, positivepiston pump 3 urges reagent 13 through three-way valve 7 and alongconduit 21 towards T-junction 29. Simultaneously, suction pump 4 drawsfluid from T-junction 29 through conduits 22 and 24, via mixer 16 toconduit 23 and via sensor 17 to conduit 26 and then into suction pump 4via three way valve 8.

The differences in flow rates between positive pump 3 and suction pump 4determine the flow rate or rates of sample 14 aspirated towardsT-junction 29.

Any flow program comprising a sequence of functional steps, each ofdefined flow rate ratio or ratios, may be chosen in accordance with theprinciples described in our U.S. Pat. No. 5,080,866, for example fixedratios or gradients.

In the sample measurement half-cycle, two-way valve 9 is open to thesample, and two-way valve 10 is closed to waste 15. As well, three-wayvalves 5 and 6 are closed to conduits 18, 19 and 20 so that piston pumps1 and 2 refill with standards 11 and 12 respectively.

The second (recalibration) half of the cycle commences with valves5,6,7,8,9 and 10 reversing their states, that is, three-way valves 5 and6 are open to conduits 18, 19 and 20; three-way valves 7 and 8 areclosed to conduits 21, 22, 23, 24 and 26; two-way valve 9 is closed tosample 14, and two-way valve 10 is opened to waste 15.

Simultaneously, piston pump 1 directs standard 11 through three-wayvalve 5 and along conduit 18 towards junction 27, while piston pump 2directs standard 12 through three-way valve 6 and along conduit 19towards junction 27. Standards 11 and 12 combine at junction 27 and thecombined stream flows along conduits 20 and 24; past mixer 16 and alongconduit 23; past sensor 17 and along conduit 25; and through two-wayvalve 10 to waste 15.

The ratio of the flow rates of pumps 1 and 2 through the recalibrationhalf-cycle can be a distinct flow ratio, a series of distinct flowratios, a distinct gradient of flow rate ratios, or a series of distinctgradients of flow rate ratios or any combination thereof. During thecalibration half-cycle, piston pump 3 refills with reagent 13 throughthree-way valve 7 while suction pump 4 discharges to waste 15 throughthree-way valve 8.

Standard 11 may be replaced by standard 31 to effect auto-ranging, thatis, the recalibration profile is altered by maintaining three-way valve5 closed during the recalibration half-cycle, and opening three-wayvalve 50 so that standard 31 travels along conduit 30 to join standard12 at junction 27. Mixer 16 should preferably be a low volume, lowdispersion type as described in our U.S. Pat. No. 5,040,898. The pistonpumps may be driven by any means capable of producing the requiredpiston movement, for example, stepper motors, cams with selectedprofiles, servo controls, or the like.

In FIG. 2, the function of pumps 1,2 and 40 is the same as in FIG. 1,but the sample measurement half-cycle involves positive pumping of thesample by piston pump 3 rather than aspiration by a differential pumpingmeans. With two-way valve 32 closing conduit 22 and two-way valve 9open, sample 14 is drawn along conduit 21 while recalibration is takingplace. During the sample measurement half-cycle, two-way valve 9 isclosed to sample 14 and two-way valve 32 opens conduit 22, so thatsample 14 is urged along conduit 24, through mixer 16; along conduit 23and through sensor 17; and along conduit 25 to waste 15.

FIG. 3 shows schematically a sensor response to a sample in the firsthalf of a pump cycle (0°-180°) and three standard gradient calibrationcurves in the second half of the cycle (180°-360°).

The sample response rises from a base line to a plateau value "h" andthen returns to the base line.

Curve S1-S6 represents a typical linear gradient calibration forstandards S1 and S6 with plateau regions for those standards on eachside of the curve.

Curve S3-S4 represents a narrow range linear gradient calibration forstandards S3 and S4, again with plateau responses corresponding to eachindividual standard. This narrow range gradient closely "brackets" thesample response value to obtain a higher degree of accuracy.

Accordingly, it can be seen that by employing an appropriate range ofstandards for calibration purposes, the apparatus and method of theinvention permit autoranging of the calibration half cycle by selectingparticular standards on the basis of the sample response value detectedin the preceding half cycle.

The calibration gradient between standards S2 and S5 demonstrates anon-linear calibration. The sample value is determined by extrapolationof the sample response value (h) across to the appropriate calibrationcurve. The intersection point (X, Y or Z) as measured by cycle angle(eg. encoder pulses) is a function of sample concentration.

FIG. 4 illustrates schematically a step profile calibration curve inwhich the plateau response values of standards S7, S8 and mixturesthereof are employed to construct a calibration curve with a lineargradient.

As with the calibration curves shown in FIG. 3, the sample responsevalue is extrapolated to the calibration curve where the intersectionpoint (X₁) is measured by cycle angle, is a function of simpleconcentration.

From experiments conducted with a flame atomic absorptionspectrophotometer, the method of stepped standard additions, illustratedby FIG. 4, produces highly repeatable quantitative analyses. Theparticular advantages which were noted included:

(a) Marked reduction in manual preparation of solutions.

(b) Continuous recalibration of the instrument.

(c) Continual monitoring of instrumental drift.

(d) Monitoring of matrix effects, both chemical and physical.

(e) Automation of the method of standard additions.

(f) Good repeatability.

(g) Ready accumulation of quality assurance data.

(h) Maintenance of high throughput, despite a marginally longer runtime, as each pump cycle produces a working curve and analyticalmeasurement. Unproductive measurements such as calibration and checkstandards are eliminated.

The classical application of the sample bracketing method illustrated byFIG. 3 involves several measurements and standard preparations for eachsample and is usually too laborious and time consuming for use inproduction analyses. Experimental data produced with the arrangement ofFIG. 2 using a flame atomic absorption spectrophotometer as the sensormeans (17) have produced analyses which are highly repeatable (<1% RSD)and show good accuracy for known samples.

The method and apparatus according to the invention permit a significantincrease in productive analysis time and a reduction in the timerequired to prepare standards yet without any compromise to accuracy andrepeatability of analytical measurements. Moreover, methods such asstandard additions and sample bracketing, previously rejected in favorof more cost effective compromises, have now become automated androutine by virtue of the present invention.

It will be readily apparent to a skilled addressee that manymodifications and variations may be made to the present inventionwithout departing from the spirit and scope thereof.

We claim:
 1. A recalibrating apparatus comprising:pump means fordelivery of a fluid sample, reagent or sample/reagent mixture to a fluidjunction; first and second piston pump fluid delivery systems fordelivery at first and second flow rates respectively of first and secondfluid standards to said fluid junction; conduit means providing fluidflow communication between said fluid junction and sensor means; mixingmeans to mix said first and second fluid standards upstream of saidsensor means relative to a fluid flow direction; sensor means responsiveto a fluid condition of said fluid sample, reagent or sample/reagentmixture and responsive to a fluid condition in said first standard, saidsecond standard or mixtures thereof; flow rate control means operativelycoupled to at least one of said first or second piston pump fluiddelivery systems for controllably varying a flow rate thereof to producea plurality of ratios of said first and second flow rates in accordancewith a sequence of functional steps, each of which steps is defined byat least one of a distinct flow rate ratio, a series of distinct flowrate ratios, a distinct gradient of flow rate ratios, or a series ofdistinct gradients of flow rate ratios; said flow rate control meansoperatively coupled to said pump means and said first and second pistonpump fluid delivery systems for operating said pump means and said firstand second delivery systems in reciprocal modes of a pumping cycle sothat in one half cycle said pump means delivers said fluid sample,reagent or sample/reagent mixture to said sensor means while said firstand second piston pump systems refill with first and second standardsrespectively, and in the other half cycle said first and second pistonpump systems deliver said first and second fluid standard and mixturesthereof to said sensor means while said pump means reloads with sampleor sample/reagent mixture; and wherein the condition of said sample orsample/reagent mixture detected by said sensor is compared to thecondition of said first and second fluid standards and/or mixturesthereof detected by said sensor to effect calibration of said sample. 2.An apparatus as claimed in claim 1 wherein the pump means for deliveryof a fluid sample or sample/reagent mixture comprises a positivedisplacement pump.
 3. An apparatus as claimed in claim 1 wherein thepump means for delivery of a fluid sample or sample/reagent mixturecomprises a differential pumping means adapted to aspirate a sampleand/or a sample/reagent mixture.
 4. An apparatus as claimed in claim 1wherein said mixing means is located in said conduit means between saidfluid junction and said sensor means.
 5. An apparatus as claimed inclaim 1 wherein said flow rate control means is adapted to producesubstantially pulse free operation of said first and second pistonpumps.
 6. An apparatus as claimed in claim 5 wherein said flow ratecontrol means comprises cam means operatively associated with said firstand second piston pumps.
 7. An apparatus as claimed in claim 5 whereinsaid flow rate control means comprises a linear actuation device orstepping motor operatively associated with said first and second pistonpumps.
 8. An apparatus as claimed in claim 1 including one or moreadditional piston pump means for delivery of a further fluid standard tosaid fluid junction.
 9. An apparatus as claimed in claim 8 includingelectronic control means to selectively actuate valves associated withsaid first, second or additional piston pump means, said electroniccontrol means being operatively coupled to said sensor means to effect,in use, autoranging of fluid standards associated with said first,second and additional piston pump means.
 10. An apparatus as claimed inclaim 1 wherein the sensor means vaporises the sample or sample/reagentmixture.
 11. An apparatus as claimed in claim 1 wherein the sensor meanscomprises an atomic absorption spectrophotometer or an inductivelycoupled plasma spectrophotometer.
 12. A method of recalibrating achemical analysis apparatus, said method comprising the stepsof:delivering a fluid sample or sample/reagent mixture via a fluidjunction to sensor means responsive to a fluid condition of said sampleor sample/reagent during a first half of an operational cycle;delivering first and second fluid standards and mixtures thereof viasaid fluid junction to said sensor means during a second half of anoperational cycle to produce a recalibration profile; controllablyvarying at least one of a flow rate of said first and second fluidstandards during said second half of the operational cycle to produce aplurality of ratios of said flow rates of said first and second fluidstandards in accordance with a sequence of functional steps each ofwhich is defined by at least one of a distinct flow rate ratio, adistinct gradient of flow rate ratios, or a series of distinct gradientsof flow rate ratios; mixing said first and second fluid standardsupstream of said sensor means relative to a direction of fluid flowthrough said sensor means during said second half of the operationalcycle; sensing a fluid condition of said sample or sample/reagentmixture arriving at said sensor means during said first half cycle andsubsequently sensing a fluid condition of said first and second fluidstandards and mixtures thereof during the second half of the operationalcycle; wherein the fluid condition of the sample or sample/reagentmixture in the first half cycle is compared to the recalibration profileconstructed from sensed fluid conditions of the first and second fluidstandards and mixtures thereof, said recalibration profile beingconstructed during said second half cycle.
 13. A method as claimed inclaim 12 wherein delivery of said fluid sample or sample/reagent mixturemay be effected by positively pumping said fluid sample orsample/reagent to said sensor means.
 14. A method as claimed in claim 12wherein delivery of said fluid sample or sample/reagent mixture to saidsensor means may be effected by aspiration of said fluid sample orsample/reagent mixture with differential pumping means.
 15. A method asclaimed in claim 12 wherein said sequence of functional steps mayinclude sequentially a distinct flow rate of said first fluid standard,a distinct gradient of flow rate ratios between said first and secondfluid standards, and a distinct flow rate of said second standard.
 16. Amethod as claimed in claim 12 wherein said sequence of functional stepsmay include sequentially, a distinct flow rate of said first fluidstandard, a series of distinct flow rate ratios between said first andsecond fluid standards, and a distinct flow rate of said second fluidstandard.
 17. A method as claimed in claim 1 wherein one or moreadditional fluid standards is selectively introduced into said sensormeans during said second half cycle to effect autoranging of said fluidstandards.